1. Field of the Invention
The present invention relates generally to an inkjet printing and, more particularly, to a two pass print mode method and apparatus for limiting wind-related print defects.
2. Description of the Related Art
Inkjet printers apply ink to a print medium, such as paper, by ejecting ink droplets from at least one printhead through a column(s) or array(s) of nozzles. The printhead is mounted on a carrier that is movable in a lateral direction across the print medium, commonly termed a unidirectional scan, and ink droplets are selectively ejected from the nozzles at corresponding ink drop placement locations. Specifically, each nozzle is associated with an actuator in the printhead that is “fired” when sufficient current passes through it, the firing causing ink within an associated ink reservoir to be ejected in droplet form from the nozzle. The printhead is moved in a series of unidirectional scans or swaths across the print medium, and between the swaths, the print medium is advanced in a longitudinal or advance direction. Since the printhead moves in a direction that is perpendicular to the advance direction of the print medium, each nozzle passes in a linear manner over the print medium. A printer controller determines which actuators will be “fired” and the proper firing sequence so that a desired image is printed on the print medium.
For a given stationary position of the print medium, printing may take place during one or more unidirectional scans of the printhead carrier. As used herein, the term “unidirectional” will refer to scanning in either, but only one, of the two possible scanning directions (left to right or right to left). Thus, bi-directional scanning refers to two successive unidirectional scans in opposite directions. The term “swath” will refer to a plurality of printing lines traced along imaginary rasters, the imaginary rasters being spaced apart in the sheet feed or advance direction. Ink droplets are deposited along the printing lines on the print medium during a particular scan of the printhead carrier by selective actuation of the individual actuators associated with individual nozzles of the printhead to expel the ink droplets.
The quality of printed images produced by an inkjet printer depends in part on the resolution of the printheads. Thus, as the market pull for inkjet printing quality to approach that of silver halide photography continues, one method to achieving that goal is to increase the vertical and horizontal resolution of the printhead. This requires changes that will decrease both the ink droplet size and nozzle-to-nozzle spacing, therefore, necessitating an increase in firing frequency of the heater resistors to achieve the same or greater throughput while maintaining the same or greater color gamut and coverage of larger droplet size printheads. The result of these changes is optimally a decrease in graininess and an increase in sharpness.
However, aerodynamic forces and fluidic interactions from neighboring nozzles more adversely affect nozzles that are spaced closer together, and whose actuators are fired at higher frequencies, compared to nozzles producing larger droplets that are spaced farther apart and whose actuators are fired at lower frequencies. The results of these aerodynamic forces and fluidic interactions are severe print quality defects such as swath contraction, non-uniform horizontal intraswath banding, and overspray.
Print quality defects associated with aerodynamic and fluidic events, commonly referred to as wind-related defects, are particularly bad in monochromatic or black only printing. This is due to the fact that black only printing modes operate at much higher duty cycles and print speeds. Wind-related defects have also been found to be present at half frequency. Half frequency printing helps to support that the wind-related defects are primarily associated with aerodynamic events, and less contingent on a fluidic event occurring at the same time. Furthermore, wind-related defects have been found to occur at half duty cycle (specifically a typical two pass printing mode that uses a checkerboard pattern). Typical two pass printing is not only half frequency, but it is also half nozzle usage.
In summary, therefore, wind-related print defects refer to print quality defects that are caused by a combination of aerodynamic and fluidic events. The main driver is currently thought to be aerodynamic forces that effect satellite formation and placement of the satellites on print media. Wind related print defects are primarily seen in black only print modes, are present in all three of the current easy to implement print methods, and comprise some of the largest hindrances to better text quality.
Thus, there is a need for an innovation that will permit continued increase in the resolution of printheads without the accompanying aerodynamic and fluidic events that produce wind-related print defects.